This invention relates to the filed of analytical instrumentation, and more particularly, to the field of ion trap mass spectrometry.
In the field of ion trap mass spectrometry, it is common to sample ions generated by an external ion source into a quadrupole ion trap mass analyzer or into an ion cyclotron resonance mass analyzer. In both cases, an ion transfer optics is used to deliver ions from an ion source into an ion trap mass analyzer. In a prior art device, the polarity of a voltage applied to each of electrodes is selected depending on the polarity of ions to be analyzed. A sample solution is introduced to a spraying device.
Ion trap mass analyzers have a finite capacity with respect to the total number of ions that can be analyzed in one cycle, so it is necessary to gate the ion source. The gating function is usually performed by pulsing voltage on one or several elements in the ion transfer optics.
In the analytical applications, the flux of the ions into an ion trap is unknown in advance since the concentration of the analyzed sample can vary. Moreover, ion trap mass analyzers are frequently connected with a sample separation technique such as gas chromatography or liquid chromatography. In this case, sample concentration is changed in time by more than three orders of magnitude. Several techniques have been developed to provide optimum ion fill factor for ion trap mass spectrometers that have to operate over a substantial range of sample concentrations.
For example, U.S. Pat. No. 5,107,109 describes a method of operating an ion trap mass spectrometer wherein the fast single MS prescan is used to evaluate the total number of ions trapped during a fixed prescan ionization time. The ionization time for the main analytical scan is adjusted based on the total number of ions trapped during the prescan. The disadvantage of this method is that it is based on the total number of ions obtained in a fast prescan even though the main scan can be a MS/MS scan, resulting in much lower number of ions left in the trap after performing the second MS step in the MS/MS sequence.
Another approach to control the number of accumulated ions is utilized in a commercially available ion trap mass spectrometer from Agilent Technologies Inc. In this method, the data obtained from the previous scan are used to predict the ionization time for the next scan. This method, theoretically, allows one to predict the ionization time for the MSn scans with more certainty, since it is based on the final value of the signal in MSn scans. However, it is difficult to implement this technique with “bright” or concentrated ion sources due to the vast differences in the optimum number of ions that should be injected into an ion trap in the different modes of operations. For example, the ionization time for the standard calibration mix in a single MS mode is typically around 100 microseconds. On the other hand the ionizations time is adjusted to 300 milliseconds while performing a sensitivity test in MS/MS mode with 10 pg of Reserpine sample. If one assumes that a 10 times “brighter” (compared to commercial), ion source is installed on the system then in MS/MS mode, i.e. full scan, the ionization time will be scaled to about 30 milliseconds, resulting in improved sensitivity in this mode. However, in the single MS mode, i.e. full scan, the ionization time also would have to be reduced to 10 microseconds, which is beyond the linearity range for most of the conventional ion optical gating schemes, thus making single MS mode non operational with the “brighter” ion source.
What is needed is a system for selectively delivering a substantially reduced quantity of ions to the ion trap of a mass spectrometer from a source of bright or concentrated ions. A reduced number of ions delivered to the ion trap would enable the ion trap to operate at optimum efficiency in single MS, MS/MS, and MSn modes for concentrated samples.